Predators and their prey

1
Predators and their prey

• Numerical response
• The change in number of predators in response to
  the change in abundance of their prey
  
• Has a stabilizing effect on prey abundance
• Functional response
• The change in feeding rate by a predator on a
  type of prey in response to a change in prey
  abundance
  
• Can have a destabilizing effect on prey
  abundance
  
• Consider bobcats and cottontails

2
Bobcats and Cottontails
3
Predator Selectivity

• Taking only old, weak, diseased, injured, or
  young
  
• Impact on prey abundance is less than one would
  expect based purely on number killed by
  predators
  
• How might predators actually help prey
  populations?
  
• Possible reduction in intraspecific competition
• Predators can allow coexistence of multiple prey
  species that would otherwise exclude each other
  
• Competitive exclusion principle

4
Competitive exclusion principle

• No two species can occupy exactly the same niche
• No two species can use the same limited resources
  in the same way at the same time
  
• If they do, one will tend to exclude the other
• They may coexist if the resource is not limiting
  or they somehow partition the resource
  
• A predator may reduce the number of the most
  abundant species, allowing the weaker competitor
  to exist (resources are no longer limiting)

5
Gause's experiments

• Competition and predation microcosms
• Paramecium and didinium
• We have a harder time studying wild vertebrates

6
Home-range and Territoriality

• Behavior may prevent overexploitation of prey
• Home range
• Area visited by an individual animal on a regular
  basis within which it obtains all of its needs
  
• Territory
• Area protected by an individual animal for its
  exclusive use
  
• Breeding, feeding, nesting, and other types
• May exclude all competitors, like species, or
  like sex only

7
Bobcats in Eastern Kentucky
8
Wolf/moose Isle Royale
9
Wolf/moose Isle Royale

• 570 km2 (220 mi2)
• logged around 1900
• All game was eliminated
• 1912 moose crossed the ice to the island
• Early-successional forest was ideal habitat
• No predators!
• Population exploded to about 3000 by 1940
• There was an obvious browse line

10
Wolf/moose Isle Royale

• Wolves crossed the ice in 1949
• Reached a steady state
• about 22 wolves and 600 moose
• approximately a 301 preypredator ratio
• Wolves killed about 150 moose/yr
• 25 of the moose population
• About 7 moose per wolf per year
• Wolves appeared to limit moose to a level below
  what food resources would allow during this period

11
Wolf/moose Isle Royale

• Territoriality and pack behavior probably
  limited wolf numbers
  
• intrinsic population control
• Wolf numbers dropped to 12 by 1992
• diseases
• distemper
• possible inbreeding
• Moose population exploded to 2500 then crashed

12
Do wolves limit the Isle Royale moose populations?

• Perhaps at some times but not others.

13
Wolves and Moose, caribou, and deer

• Based on Isle Royale data and other studies in
  Canada and Alaska
  
• Wolves limit these prey, but only when
  preypredator ratio drops due to additional
  factors

14
We need to restore ecosystem integrity

• Government kills predators in some instances
  (wolves and coyotes) and restores them in others
  (wolves, grizzly bears)
  
• Hmm
• Need to consider the ecosystem approach